88 THE FROG CHAP. 



and returns to it by the veins, and not vice versa we 

 must examine the heart itself in some detail. 



The ventricle is a hollow structure with thick spongy 

 walls and a small cavity (Fig. 22, vt), and there are two 

 perfectly distinct auricles, the right (r. au}, considerably 

 larger than the left (/. au), separated from one another 

 by a vertical partition (spt. aur). 



You have already seen that the conus arteriosus 

 arises from the right side the frog's right, not yours 

 of the base of the ventricle. A little to the left of this 

 point there is an aperture through which a bristle can 

 be passed from the ventricle into either of the auricles. 

 Both auricles, then, communicate with the ventricle, by 

 a single auricula-ventricular aperture. This is guarded 

 by two little membranous flaps (au. v. v), which spring, 

 one from the dorsal, one from the ventral edge of the 

 aperture, and hang down into the ventricle, to the walls 

 of which they are attached by little tendinous cords, 

 represented in the figure "by white streaks. Thus the 

 flaps have the character of folding doors or valves opening 

 only one way ; they readily flap backwards, i.e., into 

 the ventricle, but are prevented from flapping forwards, 

 or into the auricles, by the tendinous cords attached to 

 them. The two flaps are the auriculo-ventricular valves. 

 Their mode of action is easily understood. If the 

 auricles, being full of blood, contract and squeeze 

 themselves together, the pressure will force aside the 

 valves and allow the blood a free passage into the ven- 

 tricle. On the other hand, if the ventricle contracts, 

 the blood, getting behind the valves, will force them 

 together and close the aperture, the tendinous cords 

 preventing their being driven into the auricles by undue 

 pressure. 



In the interior of the conus is a longitudinal fold or 

 valve (l.v), which traverses it obliquely, attached to 



